No solid knowledge here, just an educated guess:
Gersende looks like a variant of Gerswinda.
If that's the case, then the name is derived from Old High
German "ger" = "spear" (compare
Gertrude) and "swinþs" / Middle High
German "swinde/swint" = "strong; impetuous".
When you compare how
Amalasuintha / Amalaswintha became
Melisende in Medieval French, well, I guess Gerswinda could have become Gersende.
By the way, apparently a
Saxon lady named Gerswinda was one of the concubines of
Charlemagne from at least the autumn of 794 to the summer of 800. She's said to be the mother to his daughter Adeltrud.
(At least according to this source:
http://books.google.de/books?id=1so18Uzc8LUC&pg=PA149&lpg=PA149&dq=gerswinda&source=bl&ots=2komGFSi40&sig=cKwpLuX6afRlzVXPXhXMWM8jaV0&hl=de&sa=X&ei=7l7-UvbXJsGbtQaNvoHYCQ&ved=0CCwQ6AEwAA#v=onepage&q=gerswinda&f=false )